A number of hydrocarbons, especially lower-boiling light hydrocarbons, in subterranean formation fluids or natural gas are known to form hydrates in conjunction with the water present in the system under a variety of conditions—particularly at a combination of low temperature and high pressure (pressure and temperature are system-specific for the formation of gas hydrates). The hydrates usually exist in solid forms that are essentially insoluble in the fluid itself. As a result, any solids in a subterranean formation or natural gas fluid are at least a nuisance for production, handling and transport of these fluids. It is not uncommon for hydrate solids (or crystals) to cause plugging and/or blockage of pipelines or transfer lines or other conduits, valves and/or safety devices and/or other equipment, resulting in shutdown, loss of production and risk of explosion or unintended release of hydrocarbons into the environment either on land or off-shore. Accordingly, hydrocarbon hydrates have been of substantial interest as well as concern to many industries, particularly the petroleum and natural gas industries.
Hydrocarbon hydrates are clathrates, and are also referred to as inclusion compounds. Clathrates are cage structures formed between a host molecule and a guest molecule. A hydrocarbon hydrate generally is composed of crystals formed by water host molecules surrounding the hydrocarbon guest molecules. The smaller or lower-boiling hydrocarbon molecules, particularly C1 (methane) to C4 hydrocarbons and their mixtures, are more problematic because it is believed that their hydrate or clathrate crystals are easier to form. For instance, it is possible for ethane to form hydrates at as high as 4° C. at a pressure of about 1 MPa. If the pressure is about 3 MPa, ethane hydrates can form at as high a temperature as 14° C. Even certain non-hydrocarbons such as carbon dioxide, nitrogen and hydrogen sulfide are known to form hydrates under the proper conditions.
There are two broad techniques used to overcome or control the hydrocarbon hydrate problems, namely the use of thermodynamic inhibitors and Low Dosage Hydrate Inhibitors (LDHIs). LDHIs are referred to as such due to the low volume required to treat production streams when compared to thermodynamic inhibitors. For the thermodynamic approach, there are a number of reported or attempted methods, including water removal, increasing temperature, decreasing pressure, addition of “antifreeze” to the fluid and/or a combination of these. The types of “antifreeze” additives or thermodynamic hydrate inhibitors (THIs) include, but are not necessarily limited to methanol, ethanol, monoethylene glycol (MEG), triethylene glycol (TEG), and combinations thereof. The LDHI approach is further split into two areas, Anti-agglomerants (AAs) and kinetic hydrate inhibitors (KHIs). AAs prevent smaller hydrocarbon hydrate crystals from agglomerating into larger ones and allow a mass of hydrates, sometimes referred to as a hydrate slurry, to be transported along the conduit. KHIs however inhibit, retard and/or prevent initial hydrocarbon hydrate crystal nucleation; and/or crystal growth. Thermodynamic and kinetic hydrate control methods may be used in conjunction.
Kinetic efforts to control hydrates have included the use of different materials as inhibitors. For instance, onium compounds with at least four carbon substituents are used to inhibit the plugging of conduits by gas hydrates. Additives such as polymers with lactam rings have also been employed to control clathrate hydrates in fluid systems. LDHIs are relatively expensive materials, and it is always advantageous to determine ways of lowering the usage levels of these hydrate inhibitors while maintaining effective hydrate inhibition.
In oilfield production applications, especially offshore applications, it is common practice to pump thermodynamic inhibitors or a combination of low dose hydrate inhibitors and thermodynamic inhibitors such as methanol or ethanol subsea to inhibit the formation of natural gas hydrates plugs. Compositions containing thermodynamic inhibitors such as methanol and/or ethanol have poor inherent lubricity properties, which mean they provide very little boundary lubrication to moving parts within the injection pumping systems that are under load. These moving parts can comprise the ball valves in check valves or the pump packing seals.
Poor lubrication may cause general wear fatigue of pump moving parts and can lead to a relatively minor problem such as reduced pumping efficiency to a worst case scenario of catastrophic pump failure. These pump failures can be costly not only in terms of pump replacement but also in terms of a lack of flow assurance which can result in a shut-in of production and costs associated with deferred production. In some hydrate inhibitor applications, lubricant oil is injected into the hydrate inhibitor formulation to help reduce pump wear. However, conventional lubricant oil is not very soluble in compositions containing alcohols such as methanol or ethanol and it does not perform very well at the low concentrations required to keep it soluble. Currently no commercial products exist for reducing friction and the associated wear in hydrate inhibitor formulations that can contain methanol and/or ethanol. Thus, there exists a need for an effective specialty chemical additive that can either be injected stand-alone or blended into the hydrate inhibitor formulation as a package.
It would be desirable in the art of pumping a hydrate inhibitor composition to provide compositions and methods for pumping such compositions so that pumping efficiency and the wear on moving parts may be improved.